The present invention relates generally to hybrid internal combustion engine and air motor systems and methods and, more particularly, to such systems and methods wherein intake and exhaust valve opening and closing timing sequences during air motor or air compressor modes of operation are selected to maximize air motor efficiency or air compressor efficiency.
Typical city driving patterns for vehicles include many accelerations and decelerations. A significant portion of fuel energy is spent in accelerating the vehicle. This energy is wasted during subsequent decelerations. A pneumatic hybrid engine captures and reuses some of this energy. During braking, the engine can utilize the braking energy to work as a compressor, pumping compressed air into an on-board tank. Later, during acceleration, the engine can be powered by the stored compressed air with or without burning diesel fuel to get up to speed or until the compressed air is depleted. The additional work performed by the compressed air permits a reduction in the quantity of fuel needed to achieve the required engine power. In this way, the engine efficiency can be increased, and the vehicle fuel economy can be improved.
It is desirable to provide a method for controlling air flow in air compressor modes and air motor modes of such pneumatic-hybrid vehicles. It is also desirable to provide a system arranged to control air flow in air compressor modes and air motor modes of such air-hybrid systems.
It is also desirable to provide a method for transitioning from air motor mode to internal combustion engine mode in a manner that minimizes changes in engine torque. It is also desirable to provide a system arranged to transition from air motor mode to internal combustion engine mode in a manner that minimizes changes in engine torque.
According to an aspect of the present invention, a method is provided for operating a hybrid internal combustion engine and air motor system comprising at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through at least one of the intake valve and the exhaust valve. The method comprises, in an air compressor mode of operation, calculating air compressor efficiency according to the following equation:
      η    ⁢                  ⁢    AC    =            efficiency      ⁢                          ⁢      of      ⁢                          ⁢      air      ⁢                          ⁢      compressor        =          -                        ⅆ          A                          ⅆ          W                    and wherein
  A  =            thermodynamic      ⁢                          ⁢      availability      ⁢                          ⁢      in      ⁢                          ⁢      reservoir        ⁢                  ⁢                  =                  (                  E          -                      E            o                          )            +                        P          o                *                  (                      V            -                          V              o                                )                    -                        T          o                *                  (                      S            -                          S              o                                )                    
dA=rate of change of A
E=Internal energy of air in reservoir=f(m, Tt)
m=mass of air in reservoir=Pt*V/(R*Tt)
Tt=Temperature of air in reservoir
Pt=Pressure of air in reservoir
V=Volume of reservoir at Pt and Tt 
R=gas constant
Eo=Internal energy of air in reservoir at ambient pressure and temperature
Po=Ambient pressure
Vo=Volume of air in reservoir at ambient pressure and temperature
To=Temperature of ambient air
S=Entropy of air in reservoir=f(Pt, Tt)
So=Entropy of air in reservoir at ambient pressure and temperature
POW=Engine brake power=N*Tb 
N=Engine speed (rev/time)
Tb=Engine brake torque
W=Engine brake work=POW dt
dW=rate of change of W
An intake and exhaust valve opening and closing timing sequence is selected for maximizing air compressor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AC                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AC                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  
According to another aspect of the present invention, a method for operating a hybrid internal combustion engine and air motor system comprising at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through the exhaust valve is provided. The method comprises, in an air motor mode of operation, calculating air motor efficiency according to the following equation:
      η    ⁢                  ⁢    AM    =            efficiency      ⁢                          ⁢      of      ⁢                          ⁢      air      ⁢                          ⁢      motor        =          -                        ⅆ          W                          ⅆ          A                    
and wherein
  A  =            thermodynamic      ⁢                          ⁢      availability      ⁢                          ⁢      in      ⁢                          ⁢      reservoir        ⁢                  ⁢                  =                  (                  E          -                      E            o                          )            +                        P          o                *                  (                      V            -                          V              o                                )                    -                        T          o                *                  (                      S            -                          S              o                                )                    
dA=rate of change of A
E=Internal energy of air in reservoir=f(m, Tt)
m=mass of air in reservoir=Pt*V/(R*Tt)
Tt=Temperature of air in reservoir
Pt=Pressure of air in reservoir
V=Volume of reservoir at Pt and Tt 
R=gas constant
Eo=Internal energy of air in reservoir at ambient pressure and temperature
Po=Ambient pressure
Vo=Volume of air in reservoir at ambient pressure and temperature
To=Temperature of ambient air
S=Entropy of air in reservoir=f(Pt, Tt)
So=Entropy of air in reservoir at ambient pressure and temperature
POW=Engine brake power=N*Tb 
N=Engine speed (rev/time)
Tb=Engine brake torque
W=Engine brake work=POW dt
dW=rate of change of W
An intake and exhaust valve opening and closing timing sequence is selected for maximizing air motor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AM                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AM                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  
According to another aspect of the present invention, a method is provided for controlling a transition from air motor operation to internal combustion operation in a hybrid internal combustion engine and air motor system. The hybrid internal combustion engine and air motor system comprises (a) a chamber including at least one intake valve, at least one outlet valve, and a drivable member, (b) an air inlet line including, upstream of the at least one intake valve, an air inlet line valve, (c) an internal combustion engine mode exhaust line comprising an internal combustion engine mode exhaust line valve downstream of the at least one exhaust valve, (d) a compressed air line comprising a compressed air line valve downstream of the at least one exhaust valve and upstream of a reservoir, and (e) an air motor mode exhaust line comprising an air motor mode exhaust valve downstream of the at least one intake valve and the air inlet line valve. The method comprises transitioning from operation in the air motor mode to operation in the internal combustion engine mode by performing the steps of a) partially opening the internal combustion engine mode exhaust line valve; b) partially closing the air motor exhaust line valve; c) partially closing the compressed air line valve; d) completely opening the air inlet line valve after performing steps a), b), and c); and e) changing an intake and exhaust valve opening and closing timing sequence from an air motor timing sequence to an internal combustion engine timing sequence.
In accordance with another aspect of the present invention, a method is provided for operating a hybrid internal combustion engine and air motor system comprising at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through at least one of the intake valve and the exhaust valve. According to the method, in an air compressor mode of operation, air compressor efficiency (nAC) is calculated, and an intake and exhaust valve opening and closing timing sequence is selected for maximizing air compressor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AC                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AC                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  
In accordance with another aspect of the present invention, a method is provided for operating a hybrid internal combustion engine and air motor system comprising at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through the exhaust valve. According to the method, in an air motor mode of operation, air motor efficiency (ηAM) is calculated, and an intake and exhaust valve opening and closing timing sequence is selected for maximizing air motor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AM                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AM                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  
In accordance with another aspect of the present invention, a hybrid internal combustion engine and air motor system is provided and comprises at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through at least one of the intake valve and the exhaust valve. The system further comprises a computer configured to calculate air compressor efficiency (ηAC) during an air compressor mode of operation and select an intake and exhaust valve opening and closing timing sequence for maximizing air compressor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AC                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AC                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  
In accordance with another aspect of the present invention, a hybrid internal combustion engine and air motor system is provided and comprises at least one chamber having a drivable member and at least one intake valve and at least one exhaust valve, and a reservoir connected to the chamber through at least one of the intake valve and the exhaust valve. The system further comprises a computer configured to calculate air motor efficiency (ηAM) during an air motor mode of operation and select an intake and exhaust valve opening and closing timing sequence for maximizing air motor efficiency so that
                    ⅆ                  (                      η            ⁢                                                  ⁢            AM                    )                            ⅆ                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      =    0    and                              ⅆ          2                ⁢                  (                      η            ⁢                                                  ⁢            AM                    )                                      ⅆ          2                ⁢                  (                      Valve            ⁢                                                  ⁢            Timing                    )                      <    0.  